Energy storage and conversion in free-piston combustion engines

Abstract

Various embodiments of the present disclosure are directed towards free-piston combustion engines. As described herein, a driver section may be provided in a free-piston combustion engine for storing energy during an expansion stroke. The driver section may be configured to store sufficient energy to perform the subsequent stroke. In some embodiments, the driver section may be configured to store sufficient energy so as to enable the engine to operate continuously across engine cycles without electrical energy input. A linear electromagnetic machine may be provided in a free-piston combustion engine for converting the kinetic energy of a piston assembly into electrical energy.

What is claimed is: 
     
       1. A free-piston combustion engine system, comprising:
 a cylinder comprising a combustion section; 
 at least one free-piston assembly in contact with the combustion section; 
 at least one driver section in contact with the at least one free-piston assembly, wherein the at least one driver section is configured to store energy from the at least one free-piston assembly during an expansion stroke of a piston cycle, wherein the at least one driver section comprises a gas spring, and wherein a spring constant of the gas spring is controllable; 
 at least one linear electromagnetic machine for directly converting between kinetic energy of the at least one free-piston assembly and electrical energy; and 
 processing circuitry that, for the purpose of avoiding net electrical energy input over a subsequent stroke of the piston cycle, causes the at least one driver section to store at least a sufficient amount of energy from the at least one free-piston assembly during the expansion stroke to perform the subsequent stroke of the piston cycle, wherein the processing circuitry causes the linear electromagnetic machine to convert at least some of the energy stored in the driver section into electrical energy during the subsequent stroke of the piston cycle. 
 
     
     
       2. The free-piston engine system of  claim 1 , wherein the expansion stroke is one of a power stroke and an intake stroke. 
     
     
       3. The free-piston engine system of  claim 1 , wherein the subsequent stroke is one of a compression stroke and an exhaust stroke. 
     
     
       4. The free-piston engine system of  claim 1 , wherein, during the subsequent stroke of the piston cycle, the linear electromagnetic machine is configured to convert approximately a difference between an amount of energy stored in the driver section during the expansion stroke and the sufficient amount of energy into electrical energy. 
     
     
       5. The free-piston engine system of  claim 1 , wherein the linear electromagnetic machine is configured to convert approximately a same amount of kinetic energy into electrical energy during the expansion stroke as during the subsequent stroke. 
     
     
       6. The free-piston engine system of  claim 1 , wherein the processing circuitry is configured to control conversion between kinetic energy of the at least one free-piston assembly and electrical energy by the linear electromagnetic machine in order to at least one of maximize engine efficiency and maximize engine power output. 
     
     
       7. The free-piston engine system of  claim 1 , wherein the processing circuitry is configured to control conversion between kinetic energy of the at least one free-piston assembly and electrical energy by the linear electromagnetic machine during the expansion stroke such that a desired amount of energy is converted to electrical energy by the linear electromagnetic machine during the subsequent stroke. 
     
     
       8. The free-piston engine system of  claim 1 , wherein the processing circuitry is configured to control conversion between kinetic energy of the at least one free-piston assembly and electrical energy by the linear electromagnetic machine during the expansion stroke and the subsequent stroke such that an approximately same amount of energy is converted to electrical energy during the expansion stroke as during the subsequent stroke. 
     
     
       9. The free-piston engine system of  claim 1 , wherein the at least one driver section comprises at least one of a passive inlet gas port and a controllable inlet gas port. 
     
     
       10. The free-piston engine system of  claim 1 , wherein a geometry of the at least one driver section is controllable. 
     
     
       11. The free-piston engine system of  claim 1 , wherein the processing circuitry is configured to control the at least one driver section in order to maximize at least one of engine efficiency and engine power. 
     
     
       12. The free-piston engine system of  claim 1 , wherein the processing circuitry is configured to control at least one of the free-piston assembly, the at least one driver section, and the linear electromagnetic machine based on at least one of position of the piston assembly, velocity of the piston assembly, acceleration of the piston assembly, temperature of the piston assembly, pressure of the combustion section, temperature of the combustion section, potential energy of the combustion section, chemical energy in the combustion section, indicated work of the combustion section, indicated efficiency of the combustion section, fuel flow rate of the combustion section, air flow rate of the combustion section, pressure in the driver section, potential energy of the driver section, temperature of gas in the driver section, indicated work of the driver section, indicated efficiency of the driver section, make-up air flow rate of the driver section, temperature of the linear electromagnetic machine, electric output, electrical efficiency, engine efficiency, engine power, previous cycle performance, environmental temperature, environmental pressure, emissions characteristics, and any combination thereof. 
     
     
       13. The free-piston engine system of  claim 1 , wherein the processing circuitry is configured to cause the driver section to store energy from the at least one free-piston assembly during the expansion stroke by controlling at least one of a force, pressure, and volume associated with the driver section. 
     
     
       14. A free-piston combustion engine system, comprising:
 a cylinder comprising a combustion section; 
 at least one free-piston assembly in contact with the combustion section; 
 at least one driver section in contact with the at least one free-piston assembly, wherein the at least one driver section is configured to store energy from the at least one free-piston assembly during an expansion stroke of a piston cycle, wherein the at least one driver section comprises a gas spring, and wherein a spring constant of the gas spring is controllable; 
 at least one linear electromagnetic machine for directly converting between kinetic energy of the at least one free-piston assembly and electrical energy; and 
 processing circuitry that necessarily causes the at least one driver section to store at least a sufficient amount of energy from the at least one free-piston assembly during the expansion stroke to perform a subsequent stroke of the piston cycle without net electrical energy input over the subsequent stroke of the piston cycle, wherein the processing circuitry causes the linear electromagnetic machine to convert at least some of the energy stored in the driver section into electrical energy during the subsequent stroke of the piston cycle. 
 
     
     
       15. The free-piston engine system of  claim 14 , wherein the expansion stroke is one of a power stroke and an intake stroke, and wherein the subsequent stroke is one of a compression stroke and an exhaust stroke. 
     
     
       16. The free-piston engine system of  claim 14 , wherein, during the subsequent stroke of the piston cycle, the linear electromagnetic machine is configured to convert approximately a difference between an amount of energy stored in the driver section during the expansion stroke and the sufficient amount of energy into electrical energy. 
     
     
       17. The free-piston engine system of  claim 14 , wherein the linear electromagnetic machine is configured to convert approximately a same amount of kinetic energy into electrical energy during the expansion stroke as during the subsequent stroke. 
     
     
       18. The free-piston engine system of  claim 14 , wherein the processing circuitry is configured to control conversion between kinetic energy of the at least one free-piston assembly and electrical energy by the linear electromagnetic machine in order to at least one of maximize engine efficiency and maximize engine power output. 
     
     
       19. The free-piston engine system of  claim 14 , wherein the processing circuitry is configured to control conversion between kinetic energy of the at least one free-piston assembly and electrical energy by the linear electromagnetic machine during the expansion stroke such that a desired amount of energy is converted to electrical energy by the linear electromagnetic machine during the subsequent stroke. 
     
     
       20. The free-piston engine system of  claim 14 , wherein the at least one driver section comprises at least one of a passive inlet gas port and a controllable inlet gas port. 
     
     
       21. The free-piston engine system of  claim 14 , wherein a geometry of the at least one driver section is controllable. 
     
     
       22. The free-piston engine system of  claim 14 , wherein the processing circuitry is configured to control at least one of the free-piston assembly, the at least one driver section, and the linear electromagnetic machine based on at least one of position of the piston assembly, velocity of the piston assembly, acceleration of the piston assembly, temperature of the piston assembly, pressure of the combustion section, temperature of the combustion section, potential energy of the combustion section, chemical energy in the combustion section, indicated work of the combustion section, indicated efficiency of the combustion section, fuel flow rate of the combustion section, air flow rate of the combustion section, pressure in the driver section, potential energy of the driver section, temperature of gas in the driver section, indicated work of the driver section, indicated efficiency of the driver section, make-up air flow rate of the driver section, temperature of the linear electromagnetic machine, electric output, electrical efficiency, engine efficiency, engine power, previous cycle performance, environmental temperature, environmental pressure, emissions characteristics, and any combination thereof. 
     
     
       23. A method of controlling a free-piston combustion engine comprising at least one free-piston assembly in contact with a respective at least one driver section comprising a gas spring, wherein a spring constant of the gas spring is controllable, and at least one linear electromagnetic machine for directly converting between kinetic energy of the at least one free-piston assembly and electrical energy, the method comprising:
 receiving at least one operating characteristic of the free-piston combustion engine; 
 processing the at least one operating characteristic, using processing circuitry, to cause the driver section to store at least a sufficient amount of energy from the at least one free-piston assembly during an expansion stroke of a piston cycle to perform a subsequent stroke of the piston cycle; 
 causing, using the processing circuitry, the subsequent stroke of the piston cycle to be performed without net electrical energy input to the engine; and 
 causing, using the processing circuitry, the linear electromagnetic machine to convert at least some of the energy of the free-piston assembly into electrical energy during the subsequent stroke. 
 
     
     
       24. The method of  claim 23 , wherein the at least one operating characteristic is selected from the group consisting of position of the piston assembly, velocity of the piston assembly, acceleration of the piston assembly, temperature of the piston assembly, pressure of the combustion section, temperature of the combustion section, potential energy of the combustion section, chemical energy in the combustion section, indicated work of the combustion section, indicated efficiency of the combustion section, fuel flow rate of the combustion section, air flow rate of the combustion section, pressure in the driver section, potential energy of the driver section, temperature of gas in the driver section, indicated work of the driver section, indicated efficiency of the driver section, make-up air flow rate of the driver section, temperature of the linear electromagnetic machine, electric output, electrical efficiency, engine efficiency, engine power, previous cycle performance, environmental temperature, environmental pressure, and emissions characteristics. 
     
     
       25. A system for controlling a free-piston combustion engine comprising at least one free-piston assembly in contact with a respective at least one driver section comprising a gas spring, wherein a spring constant of the gas spring is controllable, and at least one linear electromagnetic machine for directly converting kinetic energy of the at least one free-piston assembly into electrical energy, the system comprising:
 at least one sensor coupled to the free-piston combustion engine for outputting a respective at least one sensor signal; 
 at least one control mechanism for adjusting a respective at least one operating characteristic of the free-piston combustion engine based on a respective at least one control signal; and 
 processing circuitry that takes as input the at least one sensor signal and that outputs the at least one control signal, the processing circuitry configured to:
 process the at least one sensor signal to cause, using the control mechanism, the at least one driver section to store at least a sufficient amount of energy from the at least one free-piston assembly during the expansion stroke to perform a subsequent stroke of the piston cycle without net electrical energy input over the subsequent stroke of the piston cycle, and 
 
 cause, using the control mechanism, the linear electromagnetic machine to convert at least some of the energy of the free-piston assembly into electrical energy during the subsequent stroke. 
 
     
     
       26. The system of  claim 25 , wherein the at least one sensor is selected from the group consisting of a position sensor, a velocity sensor, an accelerometer, a temperature sensor, a pressure sensor, a flow rate sensor, a current sensor, a voltage sensor, a resistance sensor, an impedance sensor, a vibration sensor, a motion sensor, a force sensor, and an emissions sensor.